During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure may involve the use of a prosthesis that is implanted into one or more of the patient's bones. In the case of a knee replacement procedure, a tibial tray is implanted into the patient's tibia. A bearing is then secured to the tibial tray. The condyle surfaces of a replacement femoral component bear against the tibial bearing.
One type of knee prosthesis is a fixed-bearing knee prosthesis. As its name suggests, the bearing of a fixed-bearing knee prosthesis does not move relative to the tibial tray. Fixed-bearing designs are commonly used when the condition of the patient's soft tissue (i.e., knee ligaments) does not allow for the use of a knee prosthesis having a mobile bearing.
In contrast, in a mobile-bearing type of knee prosthesis, the bearing can move relative to the tibial tray. Mobile-bearing knee prostheses include so-called “rotating platform” knee prostheses, wherein the bearing can rotate about a longitudinal axis on the tibial tray.
Tibial trays are commonly made of a biocompatible metal, such as a cobalt chrome alloy or a titanium alloy.
For both fixed and mobile-bearing knee prostheses, the tibial trays may be designed to be cemented into place on the patient's tibia or alternatively may be designed for cementless fixation. Cemented fixation relies on mechanical bonds between the tibial tray and the cement as well as between the cement and the bone. Cementless implants generally have surface features that are conducive to bone ingrowth into the implant component and rely to a substantial part on this bony ingrowth for secondary fixation; primary fixation is achieved through the mechanical fit of the implant and the prepared bone.
Tibial components of both fixed and mobile-bearing and cemented and cementless knee arthroplasty systems are commonly modular components, comprising a tibial tray and a polymeric bearing carried by the tibial tray. The tibial trays commonly include features extending distally, such as pegs or stems. These extensions penetrate below the surface of the tibial plateau and stabilize the tibial tray component against movement. In cementless tibial implants, the outer surfaces of these extensions are typically porous to allow for bone ingrowth. For example, in the Zimmer Trabecular Metal Monoblock tibial trays, pegs with flat distal surfaces and hexagonal axial surfaces are formed completely of a porous metal. In such trays, bone ingrowth is likely to occur along all surfaces of the pegs, including the distal surfaces.
Femoral components of such knee prosthesis systems are also designed for either cemented or cementless fixation. For cemented fixation, the femoral component typically includes recesses or cement pockets. For cementless fixation, the femoral component is designed for primary fixation through a press-fit, and includes porous bone-engaging surfaces suitable for bone ingrowth. Both designs may include pegs designed to extend into prepared holes in the femur for stabilization of the implant.
On occasion, the primary knee prosthesis fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. When the failure is debilitating, revision surgery may be necessary. In a revision, the primary knee prosthesis (or parts of it) is removed and replaced with components of a revision prosthetic system.
When the tibial or femoral implant includes extensions (such as pegs or stems) that extend into the natural bone, a revision surgery usually requires resection of the bone in order to dislodge the extensions from the bone. This resection complicates the surgery and does not dislodge the extensions from the bone, and subsequent removal of the implant with the intact extensions often results in the removal of more of the patient's natural bone than is desirable. This removal of additional bone may further compromise the bone, increase the risk of onset of bone pathologies or abnormalities, or reduce the available healthy bone for fixation of the revision implant. Moreover, the large resection usually means that a larger orthopaedic implant is necessary to fill the space and restore the joint component to its expected geometry.
This difficulty in dislodging the primary implant components from the bones is worsened by the fact that bone also grows into the extensions. Severing these connections may be problematic since not all of these areas are easily accessible without resecting large amounts of bone.
Similar issues may be presented in other types of joint prostheses.
The assignee of the present application has filed patent applications related to the use of porous metal pegs in prostheses. These patent applications include the following: U.S. Pat. Pub. No. 20110029090 A1 entitled “Prosthesis With Modular Extensions”; U.S. Pat. Pub. No. 20110035017 A1 entitled “Prosthesis With Cut-Off Pegs And Surgical Method”; U.S. Pat. Pub. No. 20110035018 A1 entitled “Prosthesis With Composite Component”; U.S. Pat. Pub. No. 20110106268 A1 entitled “Prosthesis For Cemented Fixation And Method For Making The Prosthesis”. The disclosures of all of these patent applications are incorporated by reference herein in their entireties.
Use of the extensions disclosed, for example, in U.S. Pat. Pub. No. 20110035017, allows the surgeon to cut along the bone-engaging surface of the implant and through the porous metal extensions extending into the bone from the tibial tray platform. This can be accomplished using a bone saw to easily remove the tibial platform during revision surgery. To remove the extensions from the bone, the surgeon may then cut around the outer perimeter of each extension with a saw such as a trephine saw. Each extension may then be readily removed from the bone.
U.S. Pat. Pub. Nos. 20110106268, 20110035017, 20110029092 and 20110029090 disclose sintering porous metal pegs onto a solid metal base as well as mounting the porous metal pegs on solid metal studs without sintering (through a Morse taper connection, for example). Sintering to form a metallurgical bond can be challenging in practice, particularly if the solid metal base comprises a Co—Cr—Mo alloy and the peg comprises a titanium foam; these metals are not compatible at sintering temperatures and bonding between these metals can result in the formation of brittle intermetallic materials at the interface. These brittle intermetallic materials can have unacceptable mechanical properties. Accordingly, for some materials, a mechanical connection is preferable. And while means of mechanically connecting porous and solid metal parts of an implant component disclosed in U.S. Pat. Pub. Nos. 20110106268, 20110035017, 20110029092 and 20110029090, it is desirable to explore other options for mechanical connecting porous and solid metal parts, options that may be more cost-effective or may result in advantageous properties.